# Exercise: Joins

Twelve questions about Northwind that each need more than one table, answered with inner and outer joins, a self join, sub-queries and CASE, each with the result to expect.

Journeyman · T-SQL · SQL · Updated October 30, 2026

Real questions rarely live in one table. Who supplies each product? Which shipper delivers late? Which customers bought Chai? Each answer here needs a join, a sub-query, or both.

## Get the exercise code

Open this lesson's GitHub repository, [github.com/ask-codey/exercise-joins](https://github.com/ask-codey/exercise-joins), also linked from the **Exercise** box beside the lesson. Click **Code**, then **Download ZIP**, and unzip it into your `bootcamp` folder. Or clone it with Git.

There's one `.sql` file per question, numbered in order. Each holds the question as a comment, and an **Expect** line with the right answer's shape. Nothing here changes any data.

## What to do

Open each file in SSMS with **Northwind** chosen, write your query under the question, run it with **F5**, and compare with the **Expect** line.

| Questions | Practice |
|---|---|
| 1–5 | `INNER JOIN` across two, three and more tables, with `WHERE`, `GROUP BY` and `ORDER BY` |
| 6 | A `LEFT JOIN` to find what's missing |
| 7 | Joins and aggregates over three tables |
| 8 | A table joined to itself |
| 9–10 | Sub-queries for a value and for a list |
| 11–12 | `CASE` to count and to label |

## Hints

- **Sketch the path first.** Which table holds what you start from, which holds the answer, and which keys link them?
- **Alias every table**, and put the alias in front of every column: `p.ProductName`, `s.CompanyName`.
- **"Which ones have none?"** is a `LEFT JOIN` followed by `WHERE ... IS NULL`.
- **Shippers** are linked through `Orders.ShipVia`, which holds a `ShipperId`; the column names don't match, but the values do.
- **Test sub-queries on their own** before you put them in parentheses.

## When you're done

Every result matches its **Expect** line. Question 11 is worth a second look: one pass over 830 orders, joined to the shippers, gives a delivery report card for each.

## Submit your exercise

The exercise is how you finish this lesson. When your work is done, put it on GitHub in a repo of your own, then submit that repo's address here.

1. On GitHub, click **+**, then **New repository**. Name it (for example `exercise-joins`), choose **Public**, and click **Create repository**. Leave out the README: your folder already has one.
2. In a terminal in your exercise folder (Git Bash on Windows), send your work to your new repo. If you downloaded the ZIP:

   ```bash
   git init -b main
   git add .
   git commit -m "My solution"
   git remote add origin https://github.com/your-name/exercise-joins.git
   git push -u origin main
   ```

   If you cloned the starter instead, point your copy at your own repo, then commit and push:

   ```bash
   git remote set-url origin https://github.com/your-name/exercise-joins.git
   git add .
   git commit -m "My solution"
   git push -u origin main
   ```

3. Paste your repo's address, such as `https://github.com/your-name/exercise-joins`, into **Submit your exercise** on this page. The site checks that the repo is public and holds your commits, and then the lesson is complete.

Changed something after you submitted? Push it, and submit again: your latest submission is the one that counts.

## So what

You can now follow the keys through a database to answer questions no single table could. That's most of the SQL a developer writes, and exactly what your C# data access will send.

## Key terms

- [INNER JOIN](https://www.ask-codey.com/glossary/inner-join)
- [LEFT OUTER JOIN](https://www.ask-codey.com/glossary/left-outer-join)
- [Self Join](https://www.ask-codey.com/glossary/self-join)
- [Subquery](https://www.ask-codey.com/glossary/subquery)

## Related

- [INNER JOIN](https://www.ask-codey.com/lessons/joining-tables-in-t-sql-inner-join)
- [OUTER JOIN](https://www.ask-codey.com/lessons/joining-tables-in-t-sql-outer-join)
- [Joining Multiple Tables](https://www.ask-codey.com/lessons/joining-tables-in-t-sql-joining-multiple-tables)
- [CASE / WHEN in T-SQL](https://www.ask-codey.com/lessons/case-when-in-t-sql)

Source: https://www.ask-codey.com/lessons/exercise-joins